The invention relates to a process and a device for the measurement of the inclination of boundary areas in an optical system with respect to a reference axis.
A device of this type is disclosed in EP-A2 0,253,242. In the device described in that document, a light beam which is emitted from a light source and which is bounded by a diaphragm is collimated by an optical imaging system onto a boundary area which is to be investigated and the inclination of which in relation to a reference axis is to be determined. The wavefront, reflected by the boundary area on which it is collimated, is imaged by the same optical imaging system onto a position-sensitive photodetection system. From its position-dependent output signals, an electronic evaluation system determines the deviation of the diaphragm image reflected by the boundary area to be investigated in relation to the reference axis, and thus the inclination of the boundary area to be investigated in relation to the reference axis. In the described device, a CCD line sensor is employed as a position-sensitive photodetection system. A special threshold value circuit is provided, in order to separate the measurement signals associated with the diaphragm image from the heterodyned disturbance radiation signals.
In place of the line sensor, it is also possible to use a four-quadrant diode or a lateral diode, as is realized in the electronic auto-collimator UDT model 1000 from the company United Detector Technology.
A disadvantage in the case of the measurement of the inclination of boundary areas using auto-collimators occurs in the case of low-reflection boundary areas, since in this case the position of the diaphragm image can no longer be reliably measured, as a result of the poor contrast. Likewise, the method proves to be critical where radii centers of various boundary areas lie closely one behind the other. In these circumstances, the boundary areas with the adjacent radii centers disturb the measurement as a result of extraneous light. There is even the danger of the confusion of the boundary area to be investigated with a boundary area which is not to be investigated, i.e. an unambiguous correlation of the diaphragm reflection with a specified boundary area is not possible.